In the construction industry sheetrock (or "drywall") is frequently used to cover large wall areas. In the interest of speed it is common first to put up a large sheetrock panel, covering apertures in the wall such as electrical outlet boxes and even window and door areas, and thereafter to cut openings in the sheetrock to expose or open the electrical outlet box, window, or door. This saves substantial time in comparison to precutting the panel to fit to the edge of the opening, before fastening the panel.
Special bits are known for such cutting. They are generally used in a router operating at a speed in excess of about 20,000 rpm.
For such use the bit must be able to cut both axially through a sheetrock panel, and then cut laterally in it, i.e., in a direction perpendicular to the length of the bit, thereby to make the opening in the panel. In other words, the bit must function both as a drill and as a saw.
A conventional twist drill bit performs poorly for such use: it cuts well in the axial direction, but cuts laterally only poorly because of the flute configuration. It is convenient to use the box wall itself as a "template" for cutting an opening around a box, moving the bit sideways while guiding it along the side of the box. A conventional bit will not move smoothly around the side of an electrical outlet box or door; if guided against the box it will abrade or cut into the box wall, thereby dulling itself and marring the box or other opening. Moreover, the conventional drill configuration tends to convey powdered cut sheetrock material outwardly into the room, where it must ultimately be swept up.
The so-called "WD-5" bit once marketed by the Roto-Zip Tool Corporation of Cross Plains, WI, was specially intended for cutting sheetrock. It cut both axially and laterally and had a single flute in the form of a so-called "down" or reverse spiral. (In a conventional twist drill bit, the spiral flutes advance toward the tip in the same direction as the bit is operated, all bits conventionally being rotated in the clockwise direction.) In the WD-5 bit the spiral advanced away from the direction of rotation, that is, oppositely to the usual flute direction. However, the WD-5 bit was subject to severe abrasion when it was guided around the periphery of an outlet box. Moreover, the bit was about 1/4" in diameter and so tended to cut a wider kerf or slot than was desirable. If made in smaller diameter, shear strength was lost and the bit tended to break off too easily.
Kerf width is especially important when cutting around an electrical outlet box. The box opening is ultimately covered by a cover plate which usually extends outwardly about 1/2" beyond the periphery of the box itself. If a bit which is 1/4" wide is used to cut around the outside of the box, it can be seen that the resulting panel opening may extend almost to the edge of the cover, and in some cases the cover may have an insufficient margin of overlap to cover the opening in the panel. Moreover, if the bit is kept away from the box in order to minimize tip abrasion, the effective width of the opening it cuts is further increased.
Thus there has been a need for a bit for cutting sheetrock (including drywall, gypsum board, plaster, "Gypsonite" and "Fiberbond" composite board materials, and other similar materials), which is sufficiently strong that it can be made in a diameter as small as 1/8" without an excessive breakage rate in use, yet which can be guided along the wall of an outlet box or other surface without undue abrasion.